WASHINGTON — Genetically modified wheat created by Monsanto Co. that wasn’t approved for use turned up on an 80-acre farm in Oregon last month, threatening the outlook for U.S. exports of the grain that are the world’s largest.

A farmer attempting to kill wheat with Monsanto’s Roundup herbicide found several plants survived the weedkiller, the U.S. Department of Agriculture said Wednesday in a statement. Scientists found the wheat was a strain field-tested from 1998 to 2005 and deemed safe before St. Louis-based Monsanto, the world’s largest seedmaker, pulled the product from the regulatory approval process on concern that importers would avoid genetically modified food crops.

“I would imagine even the perception that GM wheat is out there would have some impact on our exports” with so many countries “putting their foot down on not accepting” gene-altered crops, Ryan Larsen, an assistant professor of agribusiness and applied economics at North Dakota State University in Fargo, said by telephone. “This continues that bad persona that GM crops have. It allows people to say ‘See, it’s out there and we’re not being told it’s out there.’”

Government investigators are tracking the origin of the plants and consulting with trade partners to assure them the exposure is limited and poses no threat to human health, according to Michael Firko, acting deputy administrator at the USDA’s Animal and Plant Health Inspection Service. No evidence exists that the never-approved wheat has entered the commercial food or feed supply, he said. Monsanto said there’s reason to believe the incident is highly isolated and should not concern consumers or trading partners.

“There are no food, feed, or environmental safety concerns associated with the presence of the Roundup Ready gene if it is found to be present in wheat,” Monsanto said Wednesday in a statement. “Over the past decade, an annual average of 58 million acres of wheat have been planted in the U.S. This is the first report of the Roundup Ready trait being found out of place since Monsanto’s commercial development program was discontinued nine years ago.”

Monsanto halted plans to develop modified wheat in May 2004 after the Canadian Wheat Board, then the world’s largest grain seller, said its 10 biggest red spring-wheat importers, including Japan, Britain and Malaysia, wouldn’t accept modified varieties. Italy’s biggest miller, Grandi Molini Italiani, was among buyers in Europe and Asia that refused to import modified wheat amid consumer unease over eating such products.

The location of the farm was not disclosed because of the nature of the investigation, the department said, adding that officials from Oregon, Washington and Idaho along with Monsanto and trading partners were notified before today’s announcement. Criminal violations of the Plant Protection Act may include civil penalties up to $1 million.

“We are taking this very seriously,” Firko said today in a conference call with journalists. “We have a very active investigation going on in several states in the western U.S.”

U.S. Wheat Associates and the National Association of Wheat Growers, two of the industry’s biggest trade groups, said in a joint statement they are confident in the government’s investigation.

“We will cooperate with authorities in the U.S. and international markets to understand the facts surrounding this incident and help minimize its impact,” the groups said.

There are no genetically engineered wheat varieties approved for general planting in the U.S. or elsewhere, the USDA said in a statement. Mexico, Japan and Nigeria were the three biggest buyers of U.S. wheat last year, according to the government.